The present invention relates to the transmission of microwave signals, and, more particularly, to a low noise microwave window used to transmit a microwave signal across a wall between two media.
Energy and information are often transmitted on microwave signals, both on earth and in space. In one application, signals transmitted to and from satellites in orbit are transmitted as microwaves. The use of microwaves is particularly desirable, since the microwaves can be readily modulated to carry large amounts of information at high power levels, and in addition are not blocked by cloud covers.
In many spacecraft transmitter applications, the microwave power levels are of such a magnitude that the associated voltages result in multipacting or breakdown when the units are operated in vacuum. The transmitters are therefore usually pressurized with an inert gas to increase their power handling capacity and to eliminate the possibility of multipacting. The microwave signal must be conducted from the pressurized units to the vacuum environment of the spacecraft. The devices which allow for low loss passage of the microwave signals while maintaining the pressure difference are known as microwave windows.
A conventional coaxial microwave window includes a metallic conducting rod extending from the inside of the spacecraft to the outside environment, a ceramic insulating support which holds the central rod, and an annular ring which holds the ceramic support and allows the window to be fastened into the wall of the spacecraft. The microwave window is typically joined to a pressurized device in the interior of the spacecraft. The microwave window must therefore be fabricated so that there is a pressure-tight seal across the window, between the pressurized interior and the vacuum environment of the spacecraft.
To form a reliable, long-term, pressure-tight seal, the central rod is ordinarily brazed to the ceramic support, and the ceramic support is ordinarily brazed to the metallic annular ring supporting it. The brazing of metals to non-metals such as ceramics is difficult, because braze alloys typically do not wet and bond directly to ceramics. It has therefore been necessary to develop techniques to promote such wetting, including the application of metallic interlayers which wet the ceramic and also are wet by brazing alloys.
The brazing alloy and any interlayer alloys should be non-magnetic, since the presence of magnetic materials in the microwave window can lead to magnetically induced intermodulation. Such intermodulation signals are spurious microwave signals produced by the presence of the magnetic material, and become superimposed on the microwave signal being transmitted through the microwave window. Where such intermodulation signals are produced, it is typically necessary to filter out the intermodulation products from the transmitted microwave signal using filters such as notch filters on the main transmission lines. Such notch filters can typically add as much as 50 pounds to the weight of a spacecraft, and also reduce the total available effective radiated power of the microwave signal. Those intermodulation signals that fall in the band of the microwave transmission cannot be filtered and consequently degrade the transmitted signal.
The materials system used to form brazed joints in microwave windows must therefore allow the wetting of the braze metal to the ceramic support, and should also have a system coefficient of thermal expansion intermediate between that of the materials to be brazed. Unfortunately, the known materials which meet these requirements are magnetic, so that their use results in magnetic intermodulation products being imposed upon the transmitted microwave signal, and the consequent necessity of using filters which add weight to the spacecraft.
Wetting and adhesion have sometimes been promoted by roughening the metallized surfaces prior to bonding, to promote bonding. These roughened surfaces can lead to tunnelling-induced intermodulation signals, which are also undesirable. To date, there has been proposed no material system and approach for promoting the brazing of the metallic and nonmetallic components of microwave windows which does not produce the deleterious intermodulation effects.
The efficiency of microwave windows can also be reduced by multipacting, which is the secondary emission of electrons from surfaces exposed to radio frequency fields in a vacuum environment. Electrons emitted from the metallic center conductor in a vacuum environment can impact adjacent structures, resulting in the emission of secondary electrons. The secondary electrons can then impinge upon other structure resulting in yet further electron emission. The net effect of these emissions is to add additional spurious noise to the transmitted microwave signals. In some instances multipacting may be avoided by judicious selection of dimensions and dielectric materials in microwave systems. In others, electrical system requirements dictate the use of dimensions which fall well into the multipacting range for the frequencies involved.
There therefore exists a need for an improved microwave window, wherein intermodulation and multipacting effects are reduced. Preferably, such deleterious effects would be eliminated entirely, to avoid the need for heavy, expensive filters in the microwave transmission line. Such an approach must not interfere with the basic functioning of the microwave window, and must allow the transmission of the microwave signal while maintaining the integrity of the seal between the inside of the spacecraft and its external environment. Although this background of the microwave window has been directed primarily toward spacecraft applications, the same problems can be found in other microwave applications such as waveguide microwave windows, wherein a signal must be transmitted between two environments which are sealed apart from each other. The present invention fulfills this need for an improved microwave window, and further provides related advantages.